In recent years, hybrid vehicles that use a combination of an electric motor and an engine for driving wheels have attracted attention as environmentally friendly vehicles. It has been studied to provide a hybrid vehicle configured to allow external charging. The vehicle thus configured can be charged at home or the like, and therefore will go to gas stations for refueling fewer times, which is convenient for a driver. Also, an inexpensive midnight electric power or the like can be used. Therefore, this structure can be cost-justified.
Japanese Patent Laying-Open No. 8-19114 has disclosed a hybrid vehicle having an externally chargeable battery.
When an electric vehicle or a hybrid vehicle has a battery that can be externally charged, it is desired or required to charge the battery with as much energy as possible by the external charging, i.e., the charging performed from an outside of the vehicle. For such charging, it is necessary to sense an SOC (State Of Charge) of the battery with high accuracy and thereby to prevent overcharge.
Various methods have been studied for sensing the SOC of the battery. However, it is difficult to observe directly the SOC, and it has been generally performed to estimate the SOC from externally observable voltage and/or current. However, when the changing and discharging of the battery are repeated while estimating the SOC, an error in estimation increases.